This invention relates to methods and apparatus for adjustably controlling the repositioning of non-homogeneous melt conditions across the stream of a laminar flowing fluid to a desirable circumferential position. The invention is particularly useful in cold-runner or hot-runner molding systems that flow a stream of laminar flowing material, such as molten plastic, into a single or multiple cavity mold through at least one runner flowing a non-homogeneous melt.
In multi-cavity molds, it is important for the material to be uniformly delivered to each of the cavities. However, significant shear and thermal variations are developed in a polymer melt as it flows through a runner, creating non-homogeneous melt conditions that may be asymmetrical or symmetrical across the flow stream. Such variations may result in cavity to cavity mold filling imbalances of more than 30%. These same melt variations may also cause problems while filling a given cavity within multi or single cavity molds. For example, such melt variations may create unanticipated filling patterns within a part forming cavity and affect the physical attributes of the molded part, such as shrinkage or warpage.
When molding parts using multi-cavity molds, it is standard practice to geometrically balance the runner system in order to help provide the required mold filling consistency. In geometrically balanced runners, each cavity is fed by runner sections having the same lengths, cross-sectional size and shape. The same concept of a geometrically balanced runner system may also be applied to multiple runner branches that may be feeding a single part at multiple locations. However, despite the geometrical balance, it has been observed that mold filling using this balanced runner design still results in imbalance. Specifically, as described in U.S. Pat. No. 6,077,470 to Beaumont and U.S. Pat. No. 6,503,438 to Beaumont et al., parts formed in the cavities on the inside branches are often of a different size or weight than are the parts formed in the other cavities.
In particular, it has been found that even with a geometrically balanced runner system, a flow-induced cavity filling imbalance exists. Several factors act to create this imbalance, including a non-symmetrical shear distribution developed within a laminar flowing material as it travels through the runner system. Flow imbalance can also be created by a non-symmetrical temperature distribution developed across the melt stream. Both of these non-symmetrical conditions can result in variations in the viscosity across the stream of flowing material and, in some cases, in its structure.
Various manufacturing processes and apparatus use laminar flowing material flowing through one or more tools, such as dies or molds, in the formation of products. These tools have various part forming geometries used to shape the laminar flowing materials into desired products. As used hereinafter, the term “tool” includes all of the components within the body of an entire mold or die used to produce one or more products. Normally, tools of these types are constructed of high strength materials, such as tool steels or aluminum alloys having a very high compression yield strength, so as to withstand the pressure which forces the laminar flowing material through flow paths within the tools. These flow paths are commonly referred to by terms such as channels or runners, depending on the actual manufacturing process or tool being used. The terms “runner” and “runner system” will be used hereinafter to mean a flow path through a tool for laminar flowing material.
Typical cross sectional shapes of runners include, but are not limited to, full round, half round, trapezoidal, modified trapezoidal or parabolic, and rectangular. Runners maybe solidifying or non-solidifying. For example, in thermoplastic injection molding processes, laminar flowing material in cold runners solidifies during the manufacture of products and is ejected from the tool during each cycle of the process. Whereas, hot runners are typically machined inside a block of high strength material and heated within the block so that the laminar flowing material within the hot runners remains fluid and is not ejected. Some tools may contain both hot runners and cold runners.
Manufacturing processes using tools and runner systems of the types described above include, but are not limited to, injection molding, transfer molding, blow molding and extrusion molding. The materials typically used in these processes include thermoplastics, thermosets, powdered metal and ceramics employing laminar flowing carriers, such as polymers. While this invention is useful for manufacturing methods and for apparatus which use the materials described above, this invention can be used to correct imbalances occurring in any tool in which imbalances occur in runners carrying a fluid exhibiting laminar flow and having a viscosity which is affected by shear rate (as with a non-Newtonian fluid) and/or by temperature, that is a fluid exhibiting variations in its characteristics as a result of variations in shear or flow velocity across the cross section of a runner.
Molding processes produce products by flowing laminar flowing material from a material source and through a runner system in a tool to an area or areas where the material is used to form the product. Molding processes include injection and transfer molding, in which laminar flowing material is injected under high pressure into a tool and through the runner system to a cavity or cavities in the tool (called a mold). The mold may have a single parting plane which separates two mold halves for forming molded items, or the mold may be a stack mold which has more than one parting plane, each separating a pair of mold halves. The material flows in concentric laminates through runners of whichever shape is used for a tool by following the center of the path of the runners.
Another manufacturing process using laminar flowing materials flowing in a runner system through a tool is extrusion blow molding. In the extrusion blow molding process, laminar flowing material is fed from a material source through a tool which includes a single runner or a branched runner system. After the material is fed through the runner system, it passes around a normally torpedo shaped insert near the end of the runner system which is used to form the solid stream of laminar flowing material into a tube, or profile, of material exiting the die. This tube of material is normally referred to as a parison. As the parison continues to lengthen to its desired length, it is clamped between two halves of a tool closing around it, and the tool then normally pinches off the bottom of the parison. Next, air is injected inside the tube of material, causing the material to expand against part forming walls of the tool. The material inside the tool is then cooled, solidifies, and is ejected after the tool is opened at the end of each production cycle. The tool then returns into position to grab another parison.
Yet another process using laminar flowing material flowing in a runner system through a tool is extrusion. In extrusion processes the laminar flowing material is normally, continuously fed from a material source through a die having a single runner or a branched runner system to be delivered to a part forming geometry which shapes the material as it exits at the end of the die. The extrusion process is normally referred to as a steady-state process and produces continuous shapes, or profiles, such as pipes or the coatings on electrical wires. As the laminar flowing material exits the part forming die, the material is normally drawn through a coolant, such as water, where it takes on its final shape as it solidifies.
Regardless of process and the type of molding system used, as a laminar flowing material flows through a runner, material near the perimeter of the runner experiences high shear conditions, whereas the material near the center experiences low shear conditions. These shear conditions are developed from the velocity of the flowing material relative to the stationary boundary of the flow channel and the relative velocity of the laminates of material flowing through the channel. FIG. 1 illustrates a characteristic shear rate distribution across the diameter “d” of a runner, where the magnitude of the shear rate is shown on the horizontal axis and the diameter is shown on the vertical axis. Shear rate is normally at or near zero at the outermost perimeter of a runner, is at its maximum level near the perimeter of the runner, and is then reduced to a level at or near zero in the center of the runner. As shown in the cross-section of runner 20 in FIG. 2, a predominantly high shear region 22 forms around the inner periphery of the runner while a predominantly low shear region 24 forms around the center.
Material in the high shear region 22 gains heat from friction caused by the relative velocity of the laminates as the laminar flowing material flows through the runner. This heat, and the effects of the shear on the non-Newtonian characteristics of polymers and other laminar flowing materials, normally causes the high sheared material near the perimeter of the runner to have a lower viscosity. Because of this lowered viscosity, fluid in this region flows more readily than the material in the center of the runner.
The effects on the flow of laminar flowing material and products produced by this material are dominated by the contrasts of the characteristics between the high sheared region 22 and the low sheared region 24. Initially, the high and low sheared regions of material are significantly balanced about bisecting planes 26 and 28. Thus, they are “significantly balanced” from side-to-side or across a plane at a particular location in a runner. This results in a variation of a symmetrical non-homogeneous melt condition being developed across the flow channel.
Although flow may be initially balanced as shown in FIG. 2, non-balanced conditions develop in a runner system when a first runner section, such as runner 20, branches in two or more directions. Because flow is laminar, when a branch in the runner occurs, the high sheared, hotter material along the perimeter remains in its relative outer position. However, the inner material is split and is now positioned on the opposite side of the flow channel from the high sheared hotter material. This creates a side-to-side variation between upcoming side-to-side branching runners, or in a mold cavity, where the high sheared hotter material will flow to one side and the low sheared cooler material will flow to the other side. This variation will be described briefly with respect to FIG. 3.
Runner 20 may be a sprue, which is a specially designed runner that conveys material from a material source such as an injection molding machine. Alternatively, runner 20 may be a runner at a selected location in a tool. Cross-section AA in FIG. 3A shows the initial symmetrical conditions about the planes 26 and 28, the same as are depicted in FIG. 2. As runner 20 branches in two directions, each branch 30,32 receives equal portions of high and low sheared material. The high and low sheared material on the left side of runner 20 flows to the left branching runner 30, and the high and low sheared material on the right side of runner 20 flows to the right branching runner 32. The two halves of material from runner 20 will reform to an approximate shape of the branch runners 30 and 32. Assuming the material is flowing from top to bottom of runner 20, the high and low sheared material from runner 20 will distribute itself in runners 30 and 32 in the approximate positions and shapes illustrated in section BB of runner 32, which are shown in FIG. 3B.
As can be seen in FIG. 3B, due to the laminar flowing conditions of the material, the flow of material in runner 20 causes most of the high sheared material near the periphery of runner 20 to remain as high sheared material 31 on the top side of both of the branching runners 30 and 32. The low sheared material at the center of runner 20 flows to the bottom of the branch runners 30 and 32 and is shown as low sheared material 33. The distribution of the high sheared material 31 and the low sheared material 33 in runner 32 in FIG. 3B is symmetrical about plane 34. Thus, the distribution remains significantly balanced side-to-side across plane 34, which bisects the length of runner 32, as well as bisects runner 20. However, the distribution of high sheared material 31 and the low sheared material 33 is now unbalanced from side-to-side across horizontal plane 36, which bisects runner 32 and is perpendicular to plane 34. The results is a non-homogeneous melt condition which is symmetrical across plane 34 and non-symmetrical across plane 36.
Referring once again back to FIG. 3, the branch runner 32 itself branches in two directions through runner 38, which extends toward the top of FIG. 3, and runner 40, which extends toward the bottom of FIG. 3. Due to the laminar nature of the material, most or all of the high sheared material 31 at the top of runner 32, see FIG. 3B, flows into runner 38 and primarily or solely low sheared material 33 flows into runner 40. FIG. 3C shows the high sheared material 31 at Section CC of runner 38 and FIG. 3D shows the low sheared material 33 at Section DD of runner 40. The actual distribution of the high sheared material 31 across the cross section of runner 32 in any tool will determine how much, if any, of the high sheared material flows in runner 40 and, thus, whether most or all of the high sheared material 31 flows in runner 38.
Thus, laminar flow through successive branches of a runner system has the effect of shifting the flow distribution to a symmetrically unbalanced state. This imbalance leads to problems with flow-induced cavity fill. The imbalance further results in product differences and/or differences in material from one cavity to the next in a multi-cavity mold, such as differences in viscosity, temperature, cooling rate, shrinkage, and warpage. Additionally, the imbalance increases clamp tonnage, i.e., the force at which mold portions are pressed together, necessary for the mold to absorb pressure surges caused by the imbalance and non-parallel mold filling.
FIGS. 3A-D only consider the effect of the primary runner on the distribution. Other factors also affect the distribution of the flow, including those caused by the molding machine nozzle or the mold sprue. FIG. 4A shows the positioning of the high sheared laminates in a runner as a result of shear developed solely due to sprue or nozzle effect. Here, the high sheared material is distributed on the sprue side (top) of the primary runner 32 and then onto the top inner side of the branching secondary runner 38, 40 in a region 31A. As a result, the high sheared laminates exiting the secondary branching 38, 40 have symmetry about an axis A as shown, which is at an angle relative to the vertical.
FIG. 4B shows the positioning of the high sheared laminates in the runner solely as a result of shear caused by the flow through the primary runner and the branching runner. Thus, FIG. 4B corresponds to FIG. 3C and has a high shear laminate in a region 31B that is axisymmetric as it travels along the primary runner until it branches off into the secondary runner 38, 40. At that time, the high sheared material becomes positioned on the sprue side of the secondary runner, as shown, to have symmetry with horizontal axis B.
FIG. 4C shows the resultant positioning of the high sheared laminates in the runner as a result of the combination of sprue and runner effects. This can be better visualized by FIG. 4D, which shows the additive effect of the combination. In particular, there is an overlap region 31A′B′ formed from overlapping portions of regions 31A and 31B, combined with remaining portions of regions 31A and 31B. The combination has a resultant axis of symmetry that is centered about axis C, located between axes A and B as shown. As a result, the material feeding a downstream side-to-side branching runner will receive asymmetric conditions, which will normally cause the high sheared material to feed the cavities on the sprue side of the secondary runner first.
Recently, there have been developed methods and apparatus to control a repositioning of the asymmetric conditions across the stream of a laminar flowing fluid to a desirable position. This repositioning, referred to in the art of plastics molding as melt rotation technology, has been accomplished through a wide variety of designs and methods as taught in U.S. Pat. No. 6,077,470 to Beaumont and U.S. Pat. No. 6,503,438 to Beaumont et al., both of which are hereby incorporated herein by reference in their entireties and currently marketed and licensed as incorporating Meltflipper® technology. These prior methods use laminar fluid rotation devices with fixed flow geometries to achieve a calculated repositioning or rotation of the laminar fluid stream.
Typical laminar fluid rotation devices are created by machining a desired geometry into a mold or manifold surface or into a steel insert to be fit into the mold or manifold. For example, as shown in FIG. 5, two mold insert halves 110, 120 are machined to form a transitional area between branching runners. This transitional area causes a predetermined flow geometry, achieving an elevation change and a circumferential repositioning of the laminar fluid flow.
Conventionally, a laminar fluid rotation device was designed to achieve a desired circumferential repositioning of the laminar fluid to improve mold cavity filling and melt distribution through management of flow-induced melt variations. Commonly, this means designing the runner system to achieve a desirable rotation at one or more runner intersections or inline within a runner to reposition the orientation of the laminar stream to achieve a desired distribution of material. This technology does not typically achieve a homogeneous distribution of fluid to each cavity, as may be done using a static mixer, and may not eliminate asymmetric flow conditions. Rather, the technology takes advantage of the laminar flow structure to manage the conditions at downstream branches and mold cavities through controlled rotation of the stratified laminar stream.
Various examples of conventional prior art fixed structure fluid rotation devices are shown in the cross-sectional views of FIGS. 5A-C taken along the centerline of first runner section 32. These particular examples are formed from two machined mold insert halves 110 and 120. Section 100 includes channeling formed in the lower mold half 110 that corresponds to an intersection of the first runner section 32 and second runner section 38.
Although primary sections of runners 32 and 38 are commonly round in cross-section, for ease in manufacture of the molding insert halves and to facilitate ejection of the frozen runner in cold runner systems, portions of the runner in section 100 have been formed with a U-shaped cross-section, such as that shown in FIGS. 5A-C. That is, the runners through the flow rotation device 100 have a flat upper surface as shown formed from a flat face of upper mold insert 120.
A flow diverter section 115 is provided in the flow path to force the flow upwards to make an angular elevation change into a runner region 125 of the upper mold half 120. At the end of the region 125, the region transitions into second runner section 38. This flow channel path of FIG. 5A, which achieves a full elevation change, causes the asymmetrical fluid conditions to be rotated by roughly 90° (more realistically 80°) along the circumference of the flow channel, as more fully described in the above Beaumont patents.
FIGS. 5B-C show different mold geometries that have been modified through remachining or other method to have the flow design changed to reflect a different resultant elevation change. This allows for a circumferential repositioning of the asymmetrical fluid conditions in the runner by lesser amounts. That is, by having different heights of the flow diverter sections 115′ and 115″, and thus reducing the elevation change, repositioning of the laminar fluid flow to lesser degrees can be achieved. To better illustrate how elevation change effects the circumferential repositioning of the laminar fluid, FIG. 6 shows various elevational changes and approximate resultant degree of fluid flow rotation achieved.
Use of one or more correctly designed laminar fluid rotation devices in a runner system nearly always resulted in an improved fluid flow compared to runner flow without the laminar fluid rotation devices. However, due to the complex nature of the laminar flow and the interactive influences of the sprue or previous branches on the asymmetric melt conditions, a design that achieves an optimum positioning of the non-homogeneous conditions cannot always be determined without actual molding trials or use of expensive and time-consuming simulation methods.
Accordingly, current methods of melt rotation do not always result in a complete success in positioning of non-homogeneous melt conditions through melt rotation. In view of this, modification of the fluid rotation geometry was sometimes necessary to achieve a desired flow uniformity or control. For example, it may have been initially determined that a circumferential rotation of the flow stream by about 90° was desired and an initial fluid rotation device made as shown in FIG. 5A. However, after experimentation through molding trials or the like, it is realized that this design does not achieve a desired repositioning of the melt conditions (for example, there may still be an imbalance causing one or more mold cavities to fill unevenly). As a result, it may be sometimes necessary to modify the mold components by removing the mold insert and modifying the fluid rotation geometry, typically by welding or remachining of an original geometry in the mold, mold insert or hot runner manifold to adjust the profile of the flow diverter section 115 to achieve a slight adjustment in repositioning of the fluid flow. For example, the mold insert may need to be machined to have a flow diverter section changed to that shown in either FIG. 5B or 5C to achieve a desired melt rotation with an advantageous positioning of the symmetrical or non-symmetrical melt conditions.